It is often desirable to measure the volume or level of a material within a tank or other container without opening the container or weighing the container. One avenue of measuring the volume or level of the material within a container employs thermochromic materials which change from one color to another or from opaque to transparent or vice-versa at certain transition temperatures. Since the surface of the container will have a different rate of heat transfer at the surface areas adjacent the filled volume of the container as opposed to the surface areas adjacent the unfilled volume of the container, during periods of heating or cooling, a thermochromic material can reasonably indicate the level of material in the container.
One exemplary application of a thermochromic level indicator is on a propane tank, such as for a gas grill. In such an application, a strip of thermochromic material may be adhered to the outer surface of the propane tank. The strip is then heated above the transition temperature of the thermochromic material and allowed to cool. The portion of the strip adhered to the tank surface adjacent the liquid propane in the tank will tend to cool faster than that adhered to the tank adjacent the empty volume, since the propane will act as an improved heat sink over the empty volume. Consequently, the strip adjacent the volume of liquid propane will cool through its transition temperature and change from transparent to a color before the rest of the strip, thus permitting the level of liquid propane in the tank to be ascertained until the remaining portion of the strip also cools below its transition temperature. Since the rate of heat transfer changes abruptly at the interface of the filled and empty portions of the tank, the thermochromic layer will exhibit an abrupt color difference at the interface for a period of time sufficient to determine the liquid or more dense level in the tank.
It is desirable that the thermochromic strips change states with the addition of a relatively small amount of heat. However, the strips are thus often usable only over relatively narrow ranges of ambient temperatures.
Some of these level indicators are permanently adhered to the tank wall and thus suffer from an inability to be transferred from one tank to another. Other level indicators are adhered to a magnetic strip which can be temporarily affixed to the tank. However, the magnetic strip adds more thermal mass and thus tends to reduce the accuracy of the device.